Gravity Tractor

Sooner or later, say astronomers, an asteroid will be discovered on a collision course with Earth. Humanity will then begin all-out planning to prevent an impact. But while there are already plenty of ideas about how to shove asteroids out of Earth’s way, nobody knows whether any of them would work.

To change that, two groups in the United States and Europe have been developing separate plans for robotic missions to visit nonthreatening asteroids, try to deflect their orbits, and watch how they respond.

One group is the private B612 Foundation of Tiburon, CA, which aims to launch an unmanned spacecraft sometime in the next decade to try out a new idea for diverting an asteroid without even touching it. If a spacecraft flew to an asteroid decades before its expected impact and hovered there for years – the required period would vary according to the masses of the spacecraft and the asteroid – the slight gravitational pull of the spacecraft itself might change the asteroid’s orbit enough to turn a hit into a miss, according to a study published last November in the journal Nature by NASA astronauts Stanley Love and Edward Lu, who is a member of the board of B612. They call the concept the “gravity tractor.”

The tractor could deflect an asteroid before it passes through a “keyhole,” an imaginary hoop in space through which asteroids must pass if they are to strike Earth; bypassing such a keyhole would ensure a miss. Influencing an asteroid without touching it could be an advantage, since some asteroids are thought to be loosely bound piles of rubble that could simply fall apart under the influence of a more direct push, such as a nuclear detonation.

At the same time, the European Space Agency has been planning a mission called Don Quijote, which would send two robot craft to intercept an asteroid. One would crash into it, perhaps nudging it aside, while the other would fly nearby to observe the results.

In May, the B612 researchers and the European agency began “exploratory discussions” about combining the two concepts into one cheaper mission, testing first the no-touch method and then the impact, says Rusty Schweickart, a former Apollo astronaut and one of the founders of B612.